1. Field of the Invention
The present invention relates to a display device including an element having a thin film of a chemical compound capable of electroluminescence (EL) (which film hereinafter referred to as “EL film”) (which element hereinafter referred to as “EL element”), and to a method of fabricating the display device.
EL comprises phosphorescence caused at the time of transition from a triplet excited state to a ground state and fluorescence caused at the time of transition from a singlet excited state to a ground state.
An inorganic material or an organic material may be used to form an EL film. “Organic EL film” refers to a film of an organic material formed as an EL film. “Organic EL element” refers to an EL element having an organic EL film interposed between electrodes.
In this specification, “thin-film transistor (TFT) device” refers to a semiconductor device having at least three electrodes. These electrodes comprise a gate electrode, a source electrode, and a drain electrode. Each of the source electrode and the gate electrode may also serve a wiring function in some case.
2. Description of the Related Art
Display devices using an organic EL film can be reduced in weight and thickness in comparison with conventional CRTs, and application of such display devices to various uses is being advanced. Portable telephones and personal portable information terminals (personal digital assistants: PDAs) capable of being connected to the Internet have been developed and the amount of information to be visualized on a display device has become markedly large. Under these circumstances, there is an increased demand for multicolor and/or high-resolution display devices.
As a means to achieve improved resolution of a display device, a method is adopted in which voltage is applied to an EL film by means of active elements such as thin-film transistors (TFTs).
Display devices having pixel portion formed of EL elements are of a self-light-emitting type and therefore require no such light source as a backlight used for liquid crystal display devices. EL elements are therefore considered to be a promising means for realizing lightweight thin display devices.
EL elements are ordinarily constructed in such a manner that an EL film is formed over anodes formed in correspondence with pixels, and a cathode is formed as a common electrode on the EL film. In EL elements constructed in such a manner, however, a discontinuity can be easily caused in the EL film at a side surface of the anode, because the thin EL film having a thickness of 30 to 150 nm is formed over the anode having an increased thickness of about 200 nm, which is selected to set a low resistance. If a discontinuity is caused in the EL film, the anode and the cathode are short-circuited, light emission from the corresponding EL film portion is thereby made impossible, and a black-dot defect results.
A structure such as shown in section in FIG. 18 has been proposed by considering this problem. FIG. 18 is a cross-sectional view of a conventional EL element. An end of an anode 1000 is covered with an insulating film 1001 for the purpose of preventing discontinuity in an EL film 1002 and, hence, short-circuiting between the anode and a cathode 1003. The insulating film provided at the end of the anode is generally called a bump.
The structure shown in the cross-sectional view of FIG. 18, however, entails several problems in an actual fabrication process. If the side surface of insulating film 1001 is linear as shown in FIG. 18, discontinuity is liable to occur in the EL film at a joint 1004 between the upper surface of the anode and the side surface of the insulating film. That is, failure of deposition of the material of EL film 1002 occurs in a place where the gradient of the film forming surface of the EL film changes abruptly, thereby forming a gap in the EL film. Through this gap, the anode and the cathode are short-circuited. Even if discontinuity does not occur in the EL film, the thickness of the EL film may be reduced at the joint 1004 between the upper surface of the anode and the side surface of the insulating film. In such a case, an electric field is converged at the thinner portion of the EL film, so that light emission is caused only at the thinner portion.
In a case where the cathode on the insulating film is electrically connected to a wiring below the insulating film via a contact hole formed through the insulating film, discontinuity in the cathode at the side surface of the insulating film may cause failure to apply the necessary potential for display to the cathode.
Also, discontinuity is liable to occur in the EL film and in the cathode in the vicinity 1005 of the line at which the side and upper surfaces of the insulating film 1001 meet each other. Ordinarily, the insulating film (bump) is in the from of a strip such as to cover the space between adjacent pixel portion. If, on the bump formed so as to surround one pixel portion, a discontinuity is formed in the cathode and extends continuously to form a closed curve, then the portion of the cathode within the closed curve serves no electrode function and no voltage is applied to the EL film. That is, a point defect results.
In manufacture of display devices using EL elements in which the number of pixels is increased to improve the resolution, the occurrence of point defects due to short-circuiting between the anodes and cathode or due to discontinuity in the cathode is a factor which causes a reduction in yield or in image quality, and there is an urgent need to cope with this problem. Also, convergence of the electric field due to the state of the EL film locally reduced in thickness makes the luminance of the defective pixel portion different from that of nondefective pixel portion, resulting in a reduction in viewabitity. It is also necessary to cope with this problem.